Modelling and optimization of inactivation of Lactobacillus plantarum by pulsed electric field treatment

Antibiotic's target site affects the potentiation of Lactiplantibacillus plantarum inhibition and inactivation by electroporation

Introduction

Antibiotic resistance represents a growing global threat, and thus the motivation to develop novel and combined methods of bacterial inactivation is increasing. Electroporation is a technique in which electric pulses of sufficient strength are applied to permeabilize cells, including bacteria. Combining antibacterials with electroporation is a promising strategy to potentiate their bactericidal and bacteriostatic effectiveness. This approach has already proved useful for increasing bacterial inactivation, yet most studies so far have mainly focused on the maximal achievable effects, and less on the underlying mechanisms. We recently demonstrated that in the Gram-negative (G-) bacterium Escherichia coli, electroporation potentiates antibacterials targeting the peptidoglycan wall more than those with intracellular targets. However, in Gram-positive (G+) bacteria, the wall is directly accessible from the outside, and thus the dependence of potentiation on the antibacterial's target may be rather different. Here, we compare the inactivation and growth inhibition of the G+ bacterium Lactiplantibacillus plantarum for two antibiotics with different modes of action: ampicillin (inhibits cell-wall synthesis) and tetracycline (inhibits intracellular protein synthesis).

Methods

We used antibiotic concentrations ranging from 0 to 30 × MIC (minimum inhibitory concentration that we predetermined for each antibiotic), a single 1-ms electric pulse with an amplitude from 0 to 20 kV/cm, and post-pulse pre-dilution incubation of 24 h or 1 h.

Results

Electroporation increased the inhibition and inactivation efficiency of both antibiotics, but this was more pronounced for tetracycline, with statistical significance mostly limited to 24-h incubation. In general, both inhibition and inactivation grew stronger with increasing antibiotic concentration and electric field amplitude.

Discussion

Our results indicate that electroporation potentiates inactivation of G+ bacteria to a larger extent for antibiotics that inhibit intracellular processes and require transport into the cytoplasm, and to a smaller extent for antibiotics that inhibit cell-wall synthesis. This is the inverse of the relation observed in G- bacteria, and can be explained by the difference in the envelope structure: in G- bacteria the outer membrane must be breached for wall-inhibiting antibiotics to access their target, whereas in G+ bacteria the wall is inherently accessible from the outside and permeabilization does not affect this access.

Pulsed Electric Field Treatments with Nonlethal Field Strength Alter the Properties of Bacterial Spores

ABSTRACT

Pulsed electric field (PEF) treatment, an alternative to thermal processing in the food industry, is insufficient to inactivate bacterial spores. Although spores that have been treated in this manner remain alive, specific understanding of their physiological properties is limited. The purpose of this study is to describe the morphology, viability, and germination behavior of Bacillus atrophaeus spores treated with PEF. Our findings indicate that nonlethal PEF treatment results in spore deformation, dipicolinic acid (DPA) leakage, and a shorter and more uniform germination lag time (\(\def\upalpha{\unicode[Times]{x3B1}}\)\(\def\upbeta{\unicode[Times]{x3B2}}\)\(\def\upgamma{\unicode[Times]{x3B3}}\)\(\def\updelta{\unicode[Times]{x3B4}}\)\(\def\upvarepsilon{\unicode[Times]{x3B5}}\)\(\def\upzeta{\unicode[Times]{x3B6}}\)\(\def\upeta{\unicode[Times]{x3B7}}\)\(\def\uptheta{\unicode[Times]{x3B8}}\)\(\def\upiota{\unicode[Times]{x3B9}}\)\(\def\upkappa{\unicode[Times]{x3BA}}\)\(\def\uplambda{\unicode[Times]{x3BB}}\)\(\def\upmu{\unicode[Times]{x3BC}}\)\(\def\upnu{\unicode[Times]{x3BD}}\)\(\def\upxi{\unicode[Times]{x3BE}}\)\(\def\upomicron{\unicode[Times]{x3BF}}\)\(\def\uppi{\unicode[Times]{x3C0}}\)\(\def\uprho{\unicode[Times]{x3C1}}\)\(\def\upsigma{\unicode[Times]{x3C3}}\)\(\def\uptau{\unicode[Times]{x3C4}}\)\(\def\upupsilon{\unicode[Times]{x3C5}}\)\(\def\upphi{\unicode[Times]{x3C6}}\)\(\def\upchi{\unicode[Times]{x3C7}}\)\(\def\uppsy{\unicode[Times]{x3C8}}\)\(\def\upomega{\unicode[Times]{x3C9}}\)\(\def\bialpha{\boldsymbol{\alpha}}\)\(\def\bibeta{\boldsymbol{\beta}}\)\(\def\bigamma{\boldsymbol{\gamma}}\)\(\def\bidelta{\boldsymbol{\delta}}\)\(\def\bivarepsilon{\boldsymbol{\varepsilon}}\)\(\def\bizeta{\boldsymbol{\zeta}}\)\(\def\bieta{\boldsymbol{\eta}}\)\(\def\bitheta{\boldsymbol{\theta}}\)\(\def\biiota{\\boldsymbol{\iota}}\)\(\def\bikappa{\boldsymbol{\kappa}}\)\(\def\bilambda{\boldsymbol{\lambda}}\)\(\def\\bimu{\boldsymbol{\mu}}\)\(\def\binu{\boldsymbol{\nu}}\)\(\def\bixi{\boldsymbol{\xi}}\)\(\def\biomicron{\boldsymbol{\micron}}\)\(\def\bipi{\boldsymbol{\pi}}\)\(\def\birho{\boldsymbol{\rho}}\)\(\def\bisigma{\boldsymbol{\sigma}}\)\(\def\bitau{\boldsymbol{\\tau}}\)\(\def\biupsilon{\boldsymbol{\upsilon}}\)\(\def\biphi{\boldsymbol{\phi}}\)\(\def\bichi{\boldsymbol{\chi}}\)\(\def\bipsy{\boldsymbol{\psy}}\)\(\def\biomega{\boldsymbol{\omega}}\)\(\def\bupalpha{\bf{\alpha}}\)\(\def\bupbeta{\bf{\beta}}\)\(\def\bupgamma{\bf{\gamma}}\)\(\def\bupdelta{\bf{\delta}}\)\(\def\bupvarepsilon{\bf{\varepsilon}}\)\(\def\bupzeta{\bf{\zeta}}\)\(\def\bupeta{\bf{\eta}}\)\(\def\buptheta{\bf{\theta}}\)\(\def\bupiota{\bf{\iota}}\)\(\def\bupkappa{\bf{\kappa}}\)\(\def\\buplambda{\bf{\lambda}}\)\(\def\bupmu{\bf{\mu}}\)\(\def\bupnu{\bf{\nu}}\)\(\def\bupxi{\bf{\xi}}\)\(\def\bupomicron{\bf{\micron}}\)\(\def\buppi{\bf{\pi}}\)\(\def\buprho{\bf{\rho}}\)\(\def\bupsigma{\bf{\sigma}}\)\(\def\buptau{\bf{\tau}}\)\(\def\bupupsilon{\bf{\upsilon}}\)\(\def\bupphi{\bf{\phi}}\)\(\def\bupchi{\bf{\chi}}\)\(\def\buppsy{\bf{\psy}}\)\(\def\bupomega{\bf{\omega}}\)\(\def\bGamma{\bf{\Gamma}}\)\(\def\bDelta{\bf{\Delta}}\)\(\def\bTheta{\bf{\Theta}}\)\(\def\bLambda{\bf{\Lambda}}\)\(\def\bXi{\bf{\Xi}}\)\(\def\bPi{\bf{\Pi}}\)\(\def\bSigma{\bf{\Sigma}}\)\(\def\bPhi{\bf{\Phi}}\)\(\def\bPsi{\bf{\Psi}}\)\(\def\bOmega{\bf{\Omega}}\)\({T_{{\rm{lag}}}}\)), but that there is no change in release duration (\(\Delta {T_{{\rm{release}}}}\)), germination ratio, or viability. Based on our findings, we conclude that an intact morphologic state and DPA content are not prerequisites for germination and full viability and that, in contrast to nutrient-induced germination in which initially slowly released DPA triggers subsequent germination events, leaked DPA during PEF treatment does not. Spores that have been subjected to this procedure remain dormant and preserve their full germinability. We found that PEF-treated spores respond to germinants more quickly and with less heterogeneity, possibly because the tiny cracks formed on the spore surface facilitate the germinants' access to the germination receptors situated on the spore's inner membrane. The consensus view that nonlethal PEF has less impact on spores that are still capable of forming CFUs under proper conditions is one-sided. This research advances our understanding of how spores behave following nonlethal PEF treatment and gives information on the topics of nosocomial sterilization, food safety, and public health.

HIGHLIGHTS

Effect of Pulsed Electric Field (PEF) on Bacterial Viability and Whey Protein in the Processing of Raw Milk

There is growing concern regarding the nutritional value of processed food products. Although thermal pasteurization, used in food processing, is a safe method and is widely applied in the food industry, food products lack quality and nutritional value because of the high temperatures used during pasteurization. In this study, the effect of pulsed electric field (PEF) processing on whey protein content and bacterial viability in raw milk was evaluated by changing the PEF strength and number of pulses. For comparison, traditional pasteurization techniques, such as low-temperature long-time (LTLT), ultra-high temperature (UHT), and microfiltration (MF), were also tested for total whey protein content, bacterial activity, and coliforms. We found that, after treatment with PEF, a significant decrease in total bacterial viability of 2.43 log and coliforms of 0.9 log was achieved, although undenatured whey protein content was not affected at 4.98 mg/mL. While traditional pasteurization techniques showed total bacterial inactivation, they were detrimental for whey protein content: β-lactoglobulin was not detected using HPLC in samples treated with UHT. LTLT treatment led to a significant decrease of 75% in β-lactoglobulin concentration; β-lactoglobulin content in milk samples treated with MF was the lowest compared to LTLT and UHT pasteurization, and ~10% and 27% reduction was observed.

Advantages and limitations of various treatment chamber designs for reversible and irreversible electroporation in life sciences

The fundamental mechanisms of pulsed electric fields on biological cells are not yet fully elucidated, though it is apparent that membrane electroporation plays a crucial role. Little is known about treatment-chamber-specific effects, and systematic studies are scarce. Thus, the present study evaluates the (dis-)advantages of various treatment chamber designs for liquid applications at differing scales. Three chambers, namely parallel plate microfluidic (V̇: 0.1 ml/min; titanium electrodes), co-linear meso (V̇: 5.0 ml/min; stainless steel electrodes), and co-linear macro (V̇: 83.3 ml/min; stainless steel electrodes) chambers, were studied. Electroporation effects on Escherichia coli in media with 0.1-10.0 mS/cm were evaluated by plate counts and flow cytometry at 8, 16, and 20 kV/cm. For the microfluidic chamber, predominantly irreversible electroporation (2.5 logs₁₀ reductions) was seen at 0.1 mS/cm, while high irreversible electroporation (4.2 logs₁₀ reductions) at 10.0 mS/cm was observed for the macro chamber. The meso chamber indicated a similar trend towards increased conductivity, even though only low inactivation levels were present. Variation in conductivity and electrode configuration or area likely induces effects resulting in distinct electroporation levels, as observed for the micro and macro chamber. Suitable application scenarios, depending on targeted electroporation effects, were suggested.

Reversibility of membrane permeabilization upon pulsed electric field treatment in Lactobacillus plantarum WCFS1

Pulsed electric field (PEF) treatment, or electroporation, can be used to load molecules into cells. The permeabilizing effect of the PEF treatment on the cellular membrane can be either reversible or irreversible depending on the severity of the PEF treatment conditions. The influence of PEF on the reversibility of membrane permeabilization in Lactobacillus plantarum WCFS1 by two different fluorescent staining methods was investigated in this study. Whereas staining with propidium iodide (PI) before and after PEF treatment indicated small reversible permeabilized fractions of maximum 14%, the use of a double staining method with PI and SYTOX Green suggested larger reversible permeabilized fractions up to 40% of the population. This difference shows that the choice for a fluorescent staining method affects the conclusions drawn regarding reversibility of membrane permeabilization. Additionally, the effect of PEF treatment conditions on membrane integrity was compared, indicating a relation between critical electric field strength, cell size and membrane permeabilization. Overall this study showed the possibilities and limitations of fluorescent membrane integrity staining methods for PEF studies.

Analysis of Factors Influencing the Transmembrane Voltage Induced in Filamentous Fungi by Pulsed Electric Fields

This article studies the sterilization effects of high-voltage pulsed electric field (PEF) of technology on filamentous fungi. A cell dielectric model was proposed based on the physical structure of filamentous fungi. Basic theories of the electromagnetic field were comprehensively applied, and the multiphysics field simulation software COMSOL Multiphysics was used for more detailed study. The effects of PEF treatment parameters and microbial characteristic parameters on the resulting cell membrane and nuclear membrane changes were simulated and analyzed. The results showed significant effects on the transmembrane voltage of the cell membrane and nuclear membrane from the electric field intensity, pulse duration, cell membrane thickness, superposition effect of the pulses. However, the amount of hyphae had little effect, and the number of cell nuclei and the thickness of the cell walls had almost no effect on the transmembrane voltage of the cell membranes and the nuclear membranes. The results provide theoretical support for applying high-voltage PEFs to kill fungi in practical applications.

Elimination of Bean yellow mosaic virus from infected cormels of three cultivars of gladiolus using thermo-, electro- and chemotherapy

Bean yellow mosaic virus (BYMV) is a prevalent virus and major threat to gladiolus cultivation the world over. In the gladiolus repository at CSIR-NBRI, Lucknow, several plants (82-88%) of three economically important cultivars were found infected by BYMV showing severe mosaic and stripe symptoms. Affected plants exhibit diminished quality and quantity of florets and corms, thus reducing their value. Attempts were made to eliminate BYMV from the infected gladiolus cormel explants in vitro through thermotherapy (37 °C for 30 days), chemotherapy (30 mg/L ribavirin for 30 days), and electrotherapy (30 mA for 20 min), either alone and in different combinations. The in vitro regenerated plants were free from BYMV infection when checked by RT-PCR using BYMV-specific primers. The combination of electro- and chemotherapies has given the best response as compared to other treatments. Among the individual therapies, electrotherapy (30 mA/20 min) was found to be the best for and production of BYMV-free gladiolus plants (44-46%) with moderate regeneration efficiency (54-58%) followed by chemotherapy and thermotherapy. However, the cormels obtained from a combination of electro- and chemotherapy treatment (30 mA/20 min + 30 mg/L) has given highest virus free (46-52%) and highest therapy efficiency indices (56%) as compared to other treatments. Further, these cormels showed better developed root systems and produced more cormels which were larger in size as compared to the other treatments and control when grown in tissue culture media.

IGBT-Based Pulsed Electric Fields Generator for Disinfection: Design and In Vitro Studies on Pseudomonas aeruginosa

Irreversible electroporation of cell membrane with pulsed electric fields is an emerging physical method for disinfection that aims to reduce the doses and volumes of used antibiotics for wound healing. Here we report on the design of the IGBT-based pulsed electric field generator that enabled eradication of multidrug resistant Pseudomonas aeruginosa PAO1 on the gel. Using a concentric electric configuration we determined that the lower threshold of the electric field required to kill P. aeruginosa PAO1 was 89.28 ± 12.89 V mm^-1, when 200 square pulses of 300 µs duration are delivered at 3 Hz. These parameters disinfected 38.14 ± 0.79 mm² area around the single needle electrode. This study provides a step towards the design of equipment required for multidrug-resistant bacteria disinfection in patients with pulsed electric fields.

Comparative analyses of prediction models for inactivation of Escherichia coli in carrot juice by means of pulsed electric fields

This paper reports the prediction capacity of various microbial inactivation models to prefigure the bactericidal effect using pulsed electric field (PEF) on liquid food. The aim of study was to compare the various inactivation models based on accuracy and bias factor to find out the most accurate inactivation model for Escherichia coli present in carrot juice treated with PEF. In this study, E. coli suspended in carrot juice was treated with varying pulsed electric field strength for different intervals. The obtained data were utilized for the evaluation of parameters of Bigelow, Peleg, Hülsheger and Weibull inactivation models. Furthermore, secondary models were developed for Hülsheger and Weibull to predict the microbial inactivation at any level of field strength and treatment time. The secondary model for Hülsheger exhibits 5.8% error as compared to the Weibull model having 8.5% error in prediction of death kinetics of E. coli suspended in carrot juice by means of PEF. The comparative analysis of secondary models to forecast the unknown data set unveiled the superior functioning of Hülsheger model.

Assessing the efficacy of PEF treatments for improving polyphenol extraction during red wine vinifications

The influence of the electric field intensity and pulse width on the improvement of total polyphenol index (TPI) and colour intensity (CI) during extraction in an ethanolic solution (30%) and during fermentation-maceration has been investigated in different grape varieties: Grenache from two harvesting times, Syrah and Tempranillo. The aim of this study was to develop a procedure to establish the PEF treatment conditions that cause enough permeabilization in the skin cells of different grape varieties to obtain a significant improvement in the vinification process in terms of increment on the polyphenol content or reduction of maceration time.

Results obtained in this investigation indicate that extraction of polyphenols in a solution of ethanol (30%) for 2 h could be a suitable procedure to know if the PEF technology is effective for improving extraction of polyphenols from the grapes during vinification and to determine the most suitable PEF treatment conditions to obtain this objective. Improvement in the extraction during vinification only was observed with those grapes and under treatment conditions in which the improvement of the polyphenol extraction was higher than 40%. Other interesting observation from this research is the highest efficacy of PEF when treatments of the same duration are applied using longer pulses. Therefore, in a continuous process, where the flow processed is determined by the frequency applied by the PEF generator, it is possible to increase the processing capacity of the PEF installation.

Industrial relevance

Benefits from PEF treatment of the grapes before the maceration step in the vinification process have been demonstrated. Nevertheless, the characteristics of the grapes may change in different vintages and grape varieties. Therefore, it is of high importance to be able to determine the optimum PEF conditions in order to obtain the desired benefit during the vinification. The rapid method developed permits to determine PEF process parameters before the application of the PEF treatment with the objective of facilitating the phenolic extraction and therefore, reducing the maceration time. In these cases, it would be possible to remove the skins from the rest of the wine earlier, and therefore, increase the processing capacity of the winery.

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A rapid method to determine the PEF treatment parameters for grapes was developed.

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PEF treatments permitted to reduce the maceration several days during vinification.

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Improvement in vinification was related with a 40% raise on TPI in the rapid method.

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Longer pulses showed higher efficacy in electroporating cells from the grape skin.

Inactivation of Pseudomonas putida by pulsed electric field treatment: a study on the correlation of treatment parameters and inactivation efficiency in the short-pulse range

An important issue for an economic application of the pulsed electric field treatment for bacterial decontamination of wastewater is the specific treatment energy needed for effective reduction of bacterial populations. The present experimental study performed in a field amplitude range of 40 > E > 200 kV/cm and for a suspension conductivity of 0.01 = κ(e) > 0.2 S/m focusses on the application of short pulses, 25 ns > T > 10 μs, of rectangular, bipolar and exponential shape and was made on Pseudomonas putida, which is a typical and widespread wastewater microorganism. The comparison of inactivation results with calculations of the temporal and azimuthal membrane charging dynamics using the model of Pauly and Schwan revealed that for efficient inactivation, membrane segments at the cell equator have to be charged quickly and to a sufficiently high value, on the order of 0.5 V. After fulfilling this basic condition by an appropriate choice of pulse field strength and duration, the log rate of inactivation for a given suspension conductivity of 0.2 S/m was found to be independent of the duration of individual pulses for constant treatment energy expenditure. Moreover, experimental results suggest that even pulse shape plays a minor role in inactivation efficiency. The variation of the suspension conductivity resulted in comparable inactivation performance of identical pulse parameters if the product of pulse duration and number of pulses was the same, i.e., required treatment energy can be linearly downscaled for lower conductivities, provided that pulse amplitude and duration are selected for entire membrane surface permeabilization.

Equivalent pulse parameters for electroporation

Electroporation-based applications require the use of specific pulse parameters for a successful outcome. When recommended values of pulse parameters cannot be set, similar outcomes can be obtained by using equivalent pulse parameters. We determined the relations between the amplitude and duration/number of pulses resulting in the same fraction of electroporated cells. Pulse duration was varied from 150 ns to 100 ms, and the number of pulses from 1 to 128. Fura 2-AM was used to determine electroporation of cells to Ca(2+). With longer pulses or higher number of pulses, lower amplitudes are needed for the same fraction of electroporated cells. The expression derived from the model of electroporation could describe the measured data on the whole interval of pulse durations. In a narrower range (0.1-100 ms), less complex, logarithmic or power functions could be used instead. The relation between amplitude and number of pulses could best be described with a power function or an exponential function. We show that relatively simple two-parameter power or logarithmic functions are useful when equivalent pulse parameters for electroporation are sought. Such mathematical relations between pulse parameters can be important in planning of electroporation-based treatments, such as electrochemotherapy and nonthermal irreversible electroporation.

Disintegration efficiency of pulsed electric field induced effects on onion (Allium cepa L.) tissues as a function of pulse protocol and determination of cell integrity by ¹H-NMR relaxometry

The influence of electrical pulse protocol parameters on cell rupture of onion tissues was investigated in order to improve fundamental understanding and to enhance the processing of plant tissues with pulsed electric fields (PEFs). The impact of PEF parameters on cell integrity of 20 mm dia, 4-mm thick disks of Don Victor onions (Allium cepa L.) was determined by ion leakage measurements. Electric field strength, pulse width, total pulse duration, and frequency effects were determined in relation to their effects on cell damage as a function of pulse protocol. Electric field strengths up to 500 V/cm increased the damage efficiency but there was no significant difference in efficiency beyond this field strength. Larger pulse widths increased the degree of tissue disintegration at a constant pulse number. Higher PEF efficiency was achieved with shorter pulse widths and a larger number of pulses at a constant total treatment time. Lower frequencies caused a greater degree of disintegration at constant number of pulses. ¹H-NMR experiments were performed to determine the proton relaxation components of the PEF-treated onion samples and to obtain cell damage information nondestructively. Paramagnetic ion uptake by the onion sample was used to identify different proton relaxation components. Five different proton relaxation components were observed and changes in the 2 components representing different proton environments showed high correlations with ion leakage results (R²= 0.99), indicating that T(2) distributions can be used to obtain information about cell membrane integrity in PEF-treated samples. 1H-NMR proved to be an effective method for nondestructive quantification of cell membrane rupture in onions.

Inactivation of wine spoilage yeasts Dekkera bruxellensis using low electric current treatment (LEC)

AIMS

The objective of this study was to investigate the inactivation of a selected yeast Dekkera bruxellensis strain 4481 in red wine by application of low electric current treatment (LEC).

METHODS AND RESULTS

LEC (200 mA) was applied for 60 days to a red wine, Montepulciano d'Abruzzo, in an alternative strategy to the SO(2) addition during wine storage. The LEC effect on both cell activity and microflora viability was assessed. LEC decreased significantly the survival viable cells and increased the death rate of D. bruxellensis strain 4481 yeast. A final comparison was made of the main physico-chemical parameters of the wine after the different treatments. The study suggests the importance of an appropriate LEC treatment which limits wine deterioration in terms of off-flavours synthesis.

CONCLUSIONS

The results demonstrate that the growth of undesirable Dekkera can be inhibited by low voltage treatment; LEC was shown to be useful to prevent wine spoilage and has the potential of being a concrete alternative method for controlling wine spoilage.

SIGNIFICANCE AND IMPACT OF THE STUDY

Wine spoilage can be avoided by preventing the growth of undesirable Dekkera yeasts, through the effective use of LEC in the winemaking process.

Scaling-up in industrial winemaking using low electric current as an alternative to sulfur dioxide addition

AIMS

To better understand the outcome of employing low electric current (LEC) technology as a new preservation and alternative in wine technology, and to contribute to its development. It is used in industrial-scale winemaking with commercial yeast (Saccharomyces cerevisiae) during the grape must fermentation.

METHODS AND RESULTS

LEC (200 mA, time 16 days) was applied to fresh grape must as an alternative method to the usual sulfur dioxide addition used in the industrial process; two tanks, each 30,000 l, were employed for parallel fermentations. The results show that LEC decreased the survival time and increased the death rate of apiculate yeasts, whereas it did not affect the growth and survival of S. cerevisiae. A comparison was made of the main chemical and sensory parameters of the wines obtained.

CONCLUSIONS

The results have demonstrated that the low-voltage treatment had a positive effect on the grape juice fermentation (yeast microflora) during the early stages of winemaking. SIGINIFICANCE AND IMPACT OF THE STUDY: These results could be of significant importance in developing, for 'biological wine', new winemaking technologies for an innovative control process of yeast fermentation.

Refrigerated fruit juices: quality and safety issues

Fruit juices are an important source of bioactive compounds, but techniques used for their processing and subsequent storage may cause alterations in their contents so they do not provide the benefits expected by the consumer. In recent years consumers have increasingly sought so-called "fresh" products (like fresh products), stored in refrigeration. This has led the food industry to develop alternative processing technologies to produce foods with a minimum of nutritional, physicochemical, or organoleptic changes induced by the technologies themselves. Attention has also focused on evaluating the microbiological or toxicological risks that may be involved in applying these processes, and their effect on food safety, in order to obtain safe products that do not present health risks. This concept of minimal processing is currently becoming a reality with conventional technologies (mild pasteurization) and nonthermal technologies, some recently introduced (pasteurization by high hydrostatic pressure) and some perhaps with a more important role in the future (pulsed electric fields). Nevertheless, processing is not the only factor that affects the quality of these products. It is also necessary to consider the conditions for refrigerated storage and to control time and temperature.

Changes of colour and carotenoids contents during high intensity pulsed electric field treatment in orange juices

Liquid chromatography (LC) was the method chosen to evaluate the effects of high intensity pulsed electric fields (HIPEF), with different electric field intensities (25, 30, 35 and 40 kV/cm) and different treatment times (30-340 micros), on orange juice cis/trans carotenoid contents. In parallel, a conventional heat treatment (90 degrees C, 20 s) was applied to the orange juice in order to compare the effect on the carotenoid contents. HIPEF processing of orange juice is an alternative to the thermal treatment of pasteurization, provided that it is kept refrigerated, because, when the most extreme conditions of this kind of treatment are applied, the decrease in the concentration of carotenoids with vitamin A activity is very small, and also most of the carotenoids identified have a slightly increased concentration after application of the most intense treatments, although always less than in untreated fresh juice. In any case, pasteurization treatment causes a greater decrease in the concentration of most of the carotenoids identified and the carotenoids with vitamin A activity. The total carotenoid concentration decreased by 12.6% in pasteurized orange juice with respect to untreated fresh orange juice, as opposed to decreases of 9.6%, 6.3% or 7.8% when fields of 25, 30 or 40 kV/cm were applied. Orange juice treated with HIPEF shows a greater tendency towards the colour yellow and a lesser tendency towards red with respect to untreated orange juice, while the luminance of the juice remains practically invariable. This tendency is less than in pasteurized orange juice.

High-Intensity Pulsed Electric Field Variables Affecting Staphylococcus aureus Inoculated in Milk

Staphylococcus aureus is an important milk-related pathogen that is inactivated by high-intensity pulsed electric fields (HIPEF). In this study, inactivation of Staph. aureus suspended in milk by HIPEF was studied using a response surface methodology, in which electric field intensity, pulse number, pulse width, pulse polarity, and the fat content of milk were the controlled variables. It was found that the fat content of milk did not significantly affect the microbial inactivation of Staph. aureus. A maximum value of 4.5 log reductions was obtained by applying 150 bipolar pulses of 8 mus each at 35 kV/cm. Bipolar pulses were more effective than those applied in the monopolar mode. An increase in electric field intensity, pulse number, or pulse width resulted in a drop in the survival fraction of Staph. aureus. Pulse widths close to 6.7 micros lead to greater microbial death with a minimum number of applied pulses. At a constant treatment time, a greater number of shorter pulses achieved better inactivation than those treatments performed at a lower number of longer pulses. The combined action of pulse number and electric field intensity followed a similar pattern, indicating that the same fraction of microbial death can be reached with different combinations of the variables. The behavior and relationship among the electrical variables suggest that the energy input of HIPEF processing might be optimized without decreasing the microbial death.

Carotenoid profile modification during refrigerated storage in untreated and pasteurized orange juice and orange juice treated with high-intensity pulsed electric fields

A comparative study was made of the evolution and modification of various carotenoids and vitamin A in untreated orange juice, pasteurized orange juice (90 degrees C, 20 s), and orange juice processed with high-intensity pulsed electric fields (HIPEF) (30 kV/cm, 100 micros), during 7 weeks of storage at 2 and 10 degrees C. The concentration of total carotenoids in the untreated juice decreased by 12.6% when the juice was pasteurized, whereas the decrease was only 6.7% when the juice was treated with HIPEF. Vitamin A was greatest in the untreated orange juice, followed by orange juice treated with HIPEF (decrease of 7.52%) and, last, pasteurized orange juice (decrease of 15.62%). The decrease in the concentrations of total carotenoids and vitamin A during storage in refrigeration was greater in the untreated orange juice and the pasteurized juice than in the juice treated with HIPEF. During storage at 10 degrees C, auroxanthin formed in the untreated juice and in the juice treated with HIPEF. This carotenoid is a degradation product of violaxanthin. The concentration of antheraxanthin decreased during storage, and it was converted into mutatoxanthin, except in the untreated and pasteurized orange juices stored at 2 degrees C.

Enhancing inactivation of Staphylococcus aureus in skim milk by combining high-intensity pulsed electric fields and nisin

High-intensity pulsed electric fields (HIPEF) can be used as a nonthermal preservation method that is believed to enhance the effect of nisin on microorganisms such as Staphylococcus aureus. The survival of S. aureus inoculated into skim milk and treated with nisin, with HIPEF, or with a combination of nisin-HIPEF was evaluated. Nisin dose, milk pH, and HIPEF treatment time were the controlled variables that were set up at 20 to 150 ppm, pH 5.0 to 6.8, and 240 to 2,400 micros, respectively. HIPEF strength and pulse width were kept constant at 35 kV/cm and 4 micros, respectively. No reduction in S. aureus concentration was observed in skim milk at its natural pH after treatment with nisin, but 1.1 log units were recovered after 90 min of treatment at pH 5.0 with 150 ppm nisin. A reduction in viable S. aureus counts of 0.3 and 1.0 log unit in skim milk treated with HIPEF at its natural pH was observed at 240 and 2,400 micros, respectively. The nisin-HIPEF treatment design was based on a response surface methodology. The combined effect of nisin and HIPEF was clearly synergistic. However, synergism depended on pH. A maximum microbial inactivation of 6.0 log units was observed at pH 6.8, 20 ppm nisin, and 2,400 micros of HIPEF treatment time, whereas a reduction of over 4.5 log units was achieved when pH, nisin concentration, and HIPEF treatment times were set at 5.0, 150 ppm, and 240 micros, respectively.

Effect of high-intensity pulsed electric fields processing and conventional heat treatment on orange-carrot juice carotenoids

Liquid chromatography (LC) was the method of choice for quantification of carotenoids (including geometrical isomers) to evaluate the effects of high-intensity pulsed electric field (HIPEF), a nonthermal preservation method, with different parameters (electric field intensities and treatment times), on an orange-carrot juice mixture (80:20, v/v). In parallel, a conventional heat treatment (98 degrees C, 21 s) was applied to the juice. HIPEF processing generally caused a significant increase in the concentrations of the carotenoids identified as treatment time increased. HIPEF treatment at 25 and 30 kV/cm provided a vitamin A concentration higher than that found in the pasteurized juice.